Nonaqueous electrolyte secondary battery

ABSTRACT

A nonaqueous electrolyte secondary battery is provided, including: a positive electrode ( 10 ) that includes a lithium-manganese oxide as a positive electrode active material; a negative electrode ( 20 ) that includes SiOX (0≤X&lt;2) in which at least a part of a surface is covered with carbon, or a Li—Al alloy as the negative electrode active material; and an electrolytic solution ( 50 ) that contains propylene carbonate (PC), ethylene carbonate (EC), and dimethoxy ethane (DME) as an organic solvent in a range of {PC:EC:DME}={0.5 to 1.5:0.5 to 1.5:1 to 3} in terms of a volume ratio, and at least one of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) as a supporting salt in a total amount of 0.6 to 1.4 (mol/L).

This application is a 371 application of PCT/JP2016/055709 having aninternational filing date of Feb. 25, 2016, which claims priority toJP2015-049560 filed Mar. 12, 2015, the enter contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte secondarybattery.

Priority is claimed on Japanese Patent Application No. 2015-049560,filed on Mar. 12, 2015, the content of which is incorporated herein byreference.

BACKGROUND ART

A nonaqueous electrolyte secondary battery includes, in a sealedaccommodation container, a pair of polarizable electrodes including apositive electrode and negative electrode, a separator interposedbetween the positive electrode and the negative electrode, and anelectrolytic solution impregnated into the positive electrode, thenegative electrode, and the separator, and includes a supporting saltand a nonaqueous solvent such as an organic solvent. The nonaqueouselectrolyte secondary battery has a high energy density and is light inweight. Accordingly, the nonaqueous electrolyte secondary battery hasbeen used in a power supply unit of an electronic apparatus, anelectrical power storage unit that absorbs a variation of the amount ofpower generation in a power generator, and the like.

In addition, in a nonaqueous electrolyte secondary battery in which thenegative electrode includes silicon oxide (SiO_(x)) as a negativeelectrode active material, high discharging capacity is obtained.Accordingly, the nonaqueous electrolyte secondary battery has been usedas a small-sized nonaqueous electrolyte secondary battery such as acoin-type (button-type) battery. It is known that the coin-type(button-type) nonaqueous electrolyte secondary battery has excellentcharging and discharging characteristics in a high voltage and a highenergy density, and has a long cycle lifetime, and thus reliability ishigh. According to this, the nonaqueous electrolyte battery has beenused as a backup power supply of a semiconductor memory, a backup powersupply with a timepiece function, and the like (for example, refer toPTL 1) in various small-sized electronic apparatuses such as a portabletelephone, a PDA, a portable gaming machine, and a digital still camerain the related art.

In addition, in the nonaqueous electrolyte secondary battery, an organicelectrolytic solution, in which cyclic carbonic acid ester, chaincarbonic acid ester, or a mixture thereof is used as a solvent, ismainly used as an electrolytic solution. In a nonaqueous electrolytesecondary battery described in PTL 1, as the electrolytic solution,organic solvents such as dimethyl carbonate (hereinafter, may bereferred to as “DMC”), diethyl carbonate (hereinafter, may be referredto as “DEC”), propylene carbonate (hereinafter, may be referred to as“PC”), and dimethoxy ethane (hereinafter, may be referred to as “DME”),which are chain carbonic acid esters, are exemplified.

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Application, First Publication No.2000-243449

SUMMARY OF THE INVENTION Technical Problem

In the related art, for example, in a coin-type (button-type) nonaqueouselectrolyte secondary battery used for backup of a memory in a portabletelephone, a digital still camera, and the like, −20° C. to 60° C. isset as a range of an operation guarantee temperature. On the other hand,so as to operate various apparatuses in a cool region such as ahigh-latitude area, the nonaqueous electrolyte secondary battery isrequired to operate even under a low-temperature region environmentlower than the range.

However, in the nonaqueous electrolyte secondary battery of the relatedart, for example, the viscosity of the electrolytic solution increasesunder a low-temperature environment of −30° C. to −40° C., and migrationof electric charges is hindered. Therefore, there is a problem in thatsufficient discharging capacity is not obtained.

The invention has been made in consideration of the problem, and anobject thereof is to provide a nonaqueous electrolyte secondary batterycapable of retaining sufficient discharging capacity even under alow-temperature environment and is capable of operating in a broadtemperature range.

Solution to the Problem

To solve the above-described problem, the present inventors have made athorough investigation and have repeated an experiment for securingsufficient discharging capacity when using the nonaqueous electrolytesecondary battery under a low-temperature environment. As a result, theyfound that when a composition of an organic solvent and a supportingsalt used in an electrolytic solution is adjusted and optimized, and aconfiguration of a negative electrode active material used in a negativeelectrode is optimized, it is possible to retain sufficient dischargingcapacity even under the low-temperature environment.

That is, first, the present inventors found that when a mixed solvent ofa cyclic carbonate solvent having a structure (Chemical Formula 1) and achain ether solvent having a structure (Chemical Formula 2) is used asthe organic solvent included in the electrolyte, and a mixing ratio ofthe respective solvents is adjusted, it is possible to improvelow-temperature characteristics without deteriorating capacitycharacteristics at room temperature or capacity retention rate at a hightemperature.

Here, in Chemical Formula 1, R1, R2, R3, and R4 represent any one ofhydrogen, fluorine, chlorine, an alkyl group having 1 to 3 carbon atoms,and a fluorinated alkyl group. In addition, in Chemical Formula 1, R1,R2, R3, and R4 may be the same as each other or different from eachother.

Here, in Chemical Formula 2, R7 and R8 represents any one of hydrogen,fluorine, chlorine, an alkyl group having 1 to 3 carbon atoms, and afluorinated alkyl group. In addition, R7 and R8 may be the same as eachother or different from each other.

In addition, the present inventors have repeated an additionalexperiment and investigation with respect to the cyclic carbonatesolvent (Chemical Formula 1) and a chain ether solvent (Chemical Formula2) which constitute the mixed solvent.

As a result, first, they found that when ethylene carbonate (EC) andpropylene carbonate (PC) are used as the cyclic carbonate solventexpressed by Chemical Formula 1, it is possible to retain the capacityretention rate at a high temperature in a satisfactory manner.

In addition, they found that when dimethoxy ethane (DME) is used as thechain ether solvent expressed by Chemical Formula 2, it is possible toimprove the low-temperature characteristics while securing the capacityat room temperature.

In addition, they found that when a mixing ratio of EC, PC, and DME isadjusted, an effect capable of retaining the discharging capacity alow-temperature environment is significantly obtained.

In addition, the present inventors have found that when the solvent usedin the electrolytic solution is set to the composition, and acomposition and the amount of the supporting salt are adjusted andoptimized, the above-described effect capable of retaining thedischarging capacity under a low-temperature environment issignificantly obtained.

In addition, the present inventors obtained the following finding. Inaddition to the optimization of the composition of the electrolyticsolution, when using any one material among materials including SiO_(X)(0≤X<2) in which at least a surface of particles is covered with carbon,or a Li—Al alloy as a negative electrode active material used in thenegative electrode, the above-described effect capable of retaining thedischarging capacity under the low-temperature environment is moresignificantly obtained, and an operation in a broad temperature range ispossible.

That is, according to an aspect of the invention, a nonaqueouselectrolyte secondary battery is provided, including: a positiveelectrode that includes a lithium-manganese oxide as a positiveelectrode active material; a negative electrode that includes SiO_(X)(0≤X<2) in which at least a part of a surface is covered with carbon, ora Li—Al alloy as a negative electrode active material; and anelectrolytic solution that contains propylene carbonate (PC), ethylenecarbonate (EC), and dimethoxy ethane (DME) as an organic solvent in arange of {PC:EC:DME}={0.5to 1.5:0.5 to 1.5:1 to 3} in terms of a volumeratio, and at least one of lithium bis(fluorosulfonyl)imide (LiFSI) andlithium bis(trifluoromethane sulfonyl) imide (LiTFSI) as a supportingsalt in a total amount of 0.6 to 1.4 (mol/L).

According to the aspect of the invention, first, the organic solventused in the electrolytic solution is adjusted to have theabove-described composition, and thus it is possible to prevent aviscosity of the electrolytic solution from increasing under alow-temperature environment of −30° C. to −40° C. According to this, itis possible to suppress migration of electric charges from beinghindered. As a result, discharging characteristics under thelow-temperature environment are improved, and it is possible to retainsufficient discharging capacity in a broad temperature range.

Specifically, first, when using PC and EC, which have a high dielectricconstant and high solubility for the supporting salt, as a cycliccarbonate solvent, it is possible to obtain great discharging capacity.In addition, when considering that a boiling point of PC and EC is high,even in a case of being used or stored under a high-temperatureenvironment, the electrolytic solution is less likely to volatilize.

In addition, when PC having a melting point lower than that of EC and ECare mixed and used as a cyclic carbonate solvent, it is possible toimprove low-temperature characteristics.

In addition, when using DME having a low melting point as a chain ethersolvent, the low-temperature characteristics are improved. In addition,DME has a low viscosity, and thus electrical conductivity of theelectrolytic solution is improved. In addition, DME solvates with Liions, and thus the discharging capacity of the nonaqueous electrolytesecondary battery increases.

In addition, as the supporting salt used in the electrolytic solution,when using a salt including the lithium compound in a molar ratio of therange, sufficient discharging capacity is obtained in a broadtemperature range including the low-temperature environment, and thusbattery characteristics are improved.

In addition, according to the aspect of the invention, as the negativeelectrode active material in the negative electrode, when using one orboth of SiO_(X) (0≤X<2) in which a surface is covered with carbon, orthe Li—Al alloy, conductivity of the negative electrode is improved, andan increase in internal resistance under the low-temperature environmentis suppressed. Accordingly, voltage drop at initial discharging issuppressed, and thus it is possible to further stabilize the dischargingcharacteristics.

In addition, in the nonaqueous electrolyte secondary battery configuredas described above according to the aspect of the invention, theelectrolytic solution may contain lithium bis(fluorosulfonyl)imide(LiFSI) as the supporting salt in an individual amount of 0.6 to 1.4(mol/L).

When LiFSI having excellent conductivity is set as the supporting saltused in the electrolytic solution, it is possible to suppress voltagedrop at initial discharging. According to this, it is also possible toimprove discharging characteristics under the low-temperatureenvironment, and sufficient discharging capacity is obtained in a broadtemperature range. As a result, battery characteristics are improved.

In addition, in the nonaqueous electrolyte secondary battery configuredas described above according to the aspect of the invention, thepositive electrode may include at leastLi_(1+x)Co_(y)Mn_(2−x−y)O₄(0≤x≤0.33, 0<y≤0.2) as the lithium-manganeseoxide used in the positive electrode active material.

When using the positive electrode including a compound having theabove-described composition as the lithium-manganese oxide used in thepositive electrode active material, the discharging characteristicsunder the low-temperature environment are improved, and thus sufficientdischarging capacity is obtained in a broad temperature range. As aresult, the battery characteristics are improved.

In addition, in the nonaqueous electrolyte secondary battery configuredas described above according to the aspect of the invention, capacitybalance {negative electrode capacity (mAh)/positive electrode capacity(mAh)}, which is expressed by capacity of the negative electrode andcapacity of the positive electrode, may be in a range of 1.56 to 2.51.

When the capacity balance {negative electrode capacity (mAh)/positiveelectrode capacity (mAh)} between the negative electrode and thepositive electrode is set to the above-described range, and apredetermined margin is secured for the capacity on the negativeelectrode side, even when decomposition of the negative electrode activematerial due to a battery reaction rapidly progresses, it is possible tosecure negative electrode capacity equal to or greater than a constantvalue. According to this, even when the nonaqueous electrolyte secondarybattery is stored or used for a long period of time under stricttemperature and humidity environments, the discharging capacity does notdecrease, and storage characteristics are improved.

In addition, in the nonaqueous electrolyte secondary battery configuredas described above according to the aspect of the invention, a particlesize (D50) of SiO_(X) (0≤X<2) included in the negative electrode activematerial and in which at least a part of a surface is covered withcarbon may be 0.1 to 30 μm.

When the particle size (D50) of SiO_(X) (0≤X<2) included in the negativeelectrode active material and in which at least a part of a surface iscovered with carbon is in the above-described range, even when expansionor contraction of the negative electrode occurs during charging anddischarging of the nonaqueous electrolyte secondary battery,conductivity is retained. As a result, a decrease in charging anddischarging characteristics such as cycle characteristics is suppressed.

Advantageous Effects of the Invention

According to the nonaqueous electrolyte secondary battery according tothe aspect of the invention, as described above, the composition of theorganic solvent and the supporting salt used in the electrolyticsolution is optimized. In addition, as the negative electrode activematerial in the negative electrode, when using an active materialincluding any one or both of SiO_(X) (0≤X<2) in which a surface iscovered with carbon and a Li—Al alloy, it is possible to improvedischarging characteristics under a low-temperature environment.

According to this, even when the nonaqueous electrolyte secondarybattery is used or stored under a low-temperature environment of −30° C.to −40° C., excellent discharging characteristics are obtained, and thusit is possible to retain sufficient discharging capacity in a broadtemperature range.

As a result, it is possible to provide a nonaqueous electrolytesecondary battery in which the battery characteristics do notdeteriorate even under the low-temperature environment and thusexcellent charging and discharging characteristics are obtained in abroad temperature range, and excellent storage characteristics areprovided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view schematically illustrating a nonaqueouselectrolyte secondary battery configured as a coin-type (button-type)according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, a configuration of a nonaqueous electrolyte secondarybattery according to an embodiment of the invention will be described indetail with reference to FIG. 1. Furthermore, specifically, in thenonaqueous electrolyte secondary battery described in the invention, anactive material used as a positive electrode or a negative electrode,and an electrolytic solution are accommodated in a container, but theconfiguration related to the invention is also applicable to anelectrochemical cell such as a lithium ion capacitor as an example.

<Configuration of Nonaqueous Electrolyte Secondary Battery>

A nonaqueous electrolyte secondary battery 1 according to thisembodiment as illustrated in FIG. 1 is a so-called coin-type(button-type) battery. The nonaqueous electrolyte secondary battery 1includes, in an accommodation container 2, a positive electrode 10capable of intercalating and deintercalating lithium ions, a negativeelectrode 20 capable of intercalating and deintercalating lithium ions,a separator 30 disposed between the positive electrode 10 and thenegative electrode 20, and an electrolytic solution 50 that includes atleast a supporting salt and an organic solvent.

More specifically, the nonaqueous electrolyte secondary battery 1includes the accommodation container 2. The accommodation container 2includes a bottomed cylindrical positive electrode casing 12, and acovered cylindrical (hat-shaped) negative electrode casing 22 which isfixed to an opening 12 a of the positive electrode casing 12 through agasket 40 and forms an accommodation space between the positiveelectrode casing 12 and the negative electrode casing 22. Theaccommodation space is sealed by caulking the peripheral edge of theopening 12 a of the positive electrode casing 12 to an inner side, thatis, to the negative electrode casing 22 side.

In the accommodation space sealed by the accommodation container 2, thepositive electrode 10 provided on the positive electrode casing 12 side,and the negative electrode 20 provided on the negative electrode casing22 side are disposed to face each other through the separator 30. Inaddition, the accommodation container 2 is filled with the electrolyticsolution 50. In addition, in an example illustrated in FIG. 1, lithiumfoil 60 is interposed between the negative electrode 20 and theseparator 30.

In addition, as illustrated in FIG. 1, the gasket 40 is inserted alongan inner peripheral surface of the positive electrode casing 12, and isconnected to the outer periphery of the separator 30 to support theseparator 30.

In addition, the positive electrode 10, the negative electrode 20, andthe separator 30 are impregnated with the electrolytic solution 50 inthe accommodation container 2.

In the nonaqueous electrolyte secondary battery 1 of the exampleillustrated in FIG. 1, the positive electrode 10 is electricallyconnected to an inner surface of the positive electrode casing 12through a positive electrode current collector 14, and the negativeelectrode 20 is electrically connected to an inner surface of thenegative electrode casing 22 through a negative electrode currentcollector 24. In this embodiment, the nonaqueous electrolyte secondarybattery 1 including the positive electrode current collector 14 and thenegative electrode current collector 24 as illustrated in FIG. 1 isdescribed as an example. However, there is no limitation thereto, andfor example, a configuration in which the positive electrode casing 12also serves as a positive electrode current collector and the negativeelectrode casing 22 also serves as the negative electrode currentcollector may be used.

The nonaqueous electrolyte secondary battery 1 of this embodiment isconfigured as described above, and lithium ions migrate from one side ofthe positive electrode 10 and the negative electrode 20 to the otherside thereof, and thus electric charges can be stored (charged) oremitted (discharged).

In addition, the nonaqueous electrolyte secondary battery 1 of thisembodiment includes the positive electrode 10 that includes alithium-manganese oxide as a positive electrode active material, thenegative electrode 20 that includes SiO_(X) (0≤X<2) in which at least apart of a surface is covered with carbon, or a Li—Al alloy as a negativeelectrode active material, and the electrolytic solution 50 thatcontains propylene carbonate (PC), ethylene carbonate (EC), anddimethoxy ethane (DME) as an organic solvent in a range of{PC:EC:DME}={0.5 to 1.5:0.5 to 1.5:1 to 3} in terms of a volume ratio,and at least one of lithium bis(fluorosulfonyl)imide (LiFSI) and lithiumbis(trifluoromethane sulfonyl) imide (LiTFSI) as a supporting salt in atotal amount of 0.6 to 1.4 (mol/L).

[Positive Electrode Casing and Negative Electrode Casing]

In this embodiment, the positive electrode casing 12 that constitutesthe accommodation container 2 is configured in a bottomed cylindricalshape as described above, and has the opening 12 a having a circularshape in plan view. As a material of the positive electrode casing 12, amaterial known in the related art may be used without any limitation,and examples thereof include stainless steel such as SUS329J4L andNAS64.

In addition, the negative electrode casing 22 is configured in a coveredcylindrical shape (hat shape) as described above, and has aconfiguration such that an apical end 22 a thereof is inserted into thepositive electrode casing 12 from the opening 12 a. Examples of amaterial of the negative electrode casing 22 include stainless steel,which is known in the related art, similar to the material of thepositive electrode casing 12, and for example, SUS304-BA and the likecan be used. In addition, for example, a clad material obtained bypressure-welding copper, nickel, or the like to the stainless steel canbe also used as the negative electrode casing 22.

As illustrated in FIG. 1, in a state in which the gasket 40 isinterposed between the positive electrode casing 12 and the negativeelectrode casing 22, the peripheral edge of the opening 12 a of thepositive electrode casing 12 is crimped to the negative electrode casing22 side, and thus the nonaqueous electrolyte secondary battery 1 issealed and is retained in a state in which the accommodation space isformed. Accordingly, the maximum inner diameter of the positiveelectrode casing 12 is set to a dimension greater than the maximum outerdiameter of the negative electrode casing 22.

Typically, the sheet thickness of a metal sheet used in the positiveelectrode casing 12 or the negative electrode casing 22 is approximately0.1 to 0.3 mm. For example, an average sheet thickness of the entiretyof the positive electrode casing 12 or the negative electrode casing 22may be set to approximately 0.20 mm.

In addition, in the example illustrated in FIG. 1, the apical end 22 aof the negative electrode casing 22 is set to a folded-back shape alongan outer side surface of the negative electrode casing 22, but there isno limitation thereto. For example, even in a case of using the negativeelectrode casing 22 that does not have the folded-back shape in which anend surface of the metal sheet is set as the apical end 22 a, theinvention is applicable to this case.

In addition, examples of the nonaqueous electrolyte secondary battery towhich the configuration described in detail in this embodiment isapplicable include batteries of various sizes in addition to 920 size(outer diameter ϕ of 9 mm×height of 2.0 mm), which is a typical size ofa coin-type nonaqueous electrolyte secondary battery.

[Gasket]

As illustrated in FIG. 1, the gasket 40 is formed in an annular ringshape along the inner peripheral surface of the positive electrodecasing 12, and the apical end 22 a of the negative electrode casing 22is disposed inside an annular groove 41 of the gasket 40.

In addition, for example, it is preferable that a material of the gasket40 be a resin in which a heat deformation temperature is 230° C. orhigher. When the heat deformation temperature of the resin material usedin the gasket 40 is 230° C. or higher, even when the nonaqueouselectrolyte secondary battery 1 is used or stored under ahigh-temperature environment, or even when heat generation occurs duringuse of the nonaqueous electrolyte secondary battery 1, it is possible toprevent the electrolytic solution 50 from being leaked due todeformation of the gasket.

Examples of a material of the gasket 40 include plastic resins such as apolypropylene resin (PP), polyphenyl sulfide (PPS), polyethyleneterephthalate (PET), polyamide, a liquid crystal polymer (LCP), atetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA), apolyether ether ketone resin (PEEK), a polyether nitrile resin (PEN), apolyether ketone resin (PEK), a polyarylate resin, a polybutyleneterephthalate resin (PBT), a polycyclohexane dimethylene terephthalateresin, a polyether sulfone resin (PES), a polyamino bismaleimide resin,a polyether imide resin, and a fluorine resin. Among these, it ispreferable to use the polypropylene resin in the gasket 40 whenconsidering that it is possible to prevent the gasket from being greatlydeformed during use or storage under a high-temperature environment, andthe sealing properties of the nonaqueous electrolyte secondary batteryare further improved.

In addition, in the gasket 40, a material obtained by mixing glassfiber, mica, whiskers, ceramic fine powders, and the like to theabove-described material in an addition amount of 30 mass % or less maybe appropriately used. When using this material, it is possible toprevent the gasket from being greatly deformed due to a hightemperature, and thus it is possible to prevent the electrolyticsolution 50 from being leaked.

In addition, a sealing agent may be further applied onto an inner sidesurface of the annular groove of the gasket 40. As the sealing agent,asphalt, an epoxy resin, a polyamide-based resin, a butyl rubber-basedadhesive, and the like can be used. In addition, after being appliedonto the inside of the annular groove 41, the sealing agent is dried.

In addition, the gasket 40 is interposed between the positive electrodecasing 12 and the negative electrode casing 22, and at least a part ofthe gasket 40 enters a compressed state. However, a compression rate atthis time is not particularly limited, and may be set to a range inwhich an inner side of the nonaqueous electrolyte secondary battery 1can be reliably sealed, and fracture does not occur in the gasket 40.

[Electrolytic Solution]

In the nonaqueous electrolyte secondary battery 1 of this embodiment, asthe electrolytic solution 50, an electrolytic solution including atleast an organic solvent and a supporting salt is used. In addition, inthe electrolytic solution 50 described in this embodiment, as theorganic solvent, a mixed solvent containing propylene carbonate (PC),ethylene carbonate (EC), and dimethoxy ethane (DME) in a range of{PC:EC:DME}={0.5 to 1.5:0.5 to 1.5:1 to 3} in terms of a volume ratio isused. In addition, the electrolytic solution 50 contains at least one oflithium bis(fluorosulfonyl)imide (LiFSI) and lithiumbis(trifluoromethane sulfonyl) imide (LiTFSI) as a supporting salt in atotal amount of 0.6 to 1.4 (mol/L).

Typically, the electrolytic solution 50 is obtained by dissolving thesupporting salt in a nonaqueous solvent such as the organic solvent, andcharacteristics of the electrolytic solution 50 are determined inconsideration of heat resistance, a viscosity, and the like required forthe electrolytic solution 50.

Generally, in a case of using the electrolytic solution containing theorganic solvent in the nonaqueous electrolyte secondary battery,temperature dependency of conductivity increases when considering thatsolubility for a lithium salt is deficient, and thus there is a problemin that characteristics at a low temperature greatly decrease incomparison to characteristics at room temperature. On the other hand, toimprove low-temperature characteristics, for example, in a case of usingethyl methyl carbonate or acetic acid esters which have an asymmetricstructure and are chain carbonic acid esters as the organic solvent ofthe electrolytic solution, there is a problem in that thecharacteristics of the nonaqueous electrolyte secondary battery at ahigh temperature conversely deteriorate. In addition, even in a case ofusing an organic solvent such as ethyl methyl carbonate in theelectrolytic solution, solubility for a lithium salt is also deficient,and there is a limit to improvement of the low-temperaturecharacteristics.

In contrast, in this embodiment, the organic solvent used in theelectrolytic solution 50 is set to a mixed solvent that contains PC andEC, which are cyclic carbonate solvents, and DME, which is a chain ethersolvent, in a mixing ratio set to an appropriate range. Accordingly, itis possible to realize the nonaqueous electrolyte secondary battery 1capable of retaining sufficient discharging capacity in a broadtemperature range including the low-temperature environment.

Specifically, first, when PC and EC, which have a high dielectricconstant and high solubility for the supporting salt, are used as thecyclic carbonate solvent, discharging capacity of the nonaqueouselectrolyte secondary battery 1 becomes large. In addition, whenconsidering that a boiling point of PC and EC is high, even when beingused or stored under a high-temperature environment, the electrolyticsolution is less likely to volatilize.

In addition, when PC, which has a melting point lower than that of EC,and EC are mixed and used as the cyclic carbonate solvent, it ispossible to improve low-temperature characteristics.

In addition, when DME having a low melting point is used as the chainether solvent, the low-temperature characteristics are improved. Inaddition, DME has a low viscosity, and thus electrical conductivity ofthe electrolytic solution is improved. In addition, DME solvates with Liions, and thus it is possible to obtain large discharging capacity asthe nonaqueous electrolyte secondary battery.

The cyclic carbonate solvent has a structure expressed by ChemicalFormula 1, and examples thereof include propylene carbonate (PC),ethylene carbonate (EC), butylene carbonate (BC), trifluoroethylenecarbonate (TFPC), chloroethylene carbonate (CLEC), trifluoroethylenecarbonate (TFEC), difluoroethylene carbonate (DFEC), vinylene carbonate(VEC), and the like. In the nonaqueous electrolyte secondary battery 1according to the invention, particularly from the viewpoint ofimprovement of capacity retention rate at a high temperature in additionto the viewpoint of ease of film formation on an electrode onto thenegative electrode 20 and improvement of the low-temperaturecharacteristics, two kinds including PC and EC are used as the cycliccarbonate solvent having the structure expressed by Chemical Formula 1.

Here, in Chemical Formula 1, R1, R2, R3, and R4 represent any one ofhydrogen, fluorine, chlorine, an alkyl group having 1 to 3 carbon atoms,and a fluorinated alkyl group. In addition, in Chemical Formula 1, R1,R2, R3, and R4 may be the same as each other or different from eachother.

In this embodiment, as described above, when using PC and EC, which havea high dielectric constant and high solubility, for the supporting saltas the cyclic carbonate solvent, it is possible to obtain largedischarging capacity. In addition, the electrolytic solution is lesslikely to volatilize even when being used or stored under ahigh-temperature environment when considering that PC and EC have a highboiling point. In addition, when PC, which has a melting point lowerthan that of EC, and EC are mixed and used as the cyclic carbonatesolvent, it is possible to obtain excellent low-temperaturecharacteristics.

The chain ether solvent has a structure expressed by Chemical Formula 2,and examples thereof include 1,2-dimethoxy ethane (DME), 1,2-diethoxyethane (DEE), and the like.

In this embodiment, DME, which is likely to solvate with lithium ions,is used as the chain ether solvent having the structure expressed byChemical Formula 2 from the viewpoint of improving the low-temperaturecharacteristics while securing capacity at room temperature in additionto the viewpoint of improving conductivity.

Here, in Chemical Formula 2, R5 and R6 represent any one of hydrogen,fluorine, chlorine, an alkyl group having 1 to 3 carbon atoms, and afluorinated alkyl group. In addition, R5 and R6 may be the same as eachother or different from each other.

In this embodiment, as described above, when DME having a low meltingpoint is used as the chain ether solvent, the low-temperaturecharacteristics are improved. In addition, DME has a low viscosity, andthus electrical conductivity of the electrolytic solution is improved.In addition, DME solvates with Li ions, and thus it is possible toobtain large discharging capacity as the nonaqueous electrolytesecondary battery.

In this embodiment, a mixing ratio of respective organic solvents in thesolvent of the electrolytic solution 50 is set to a range of{PC:EC:DME}={0.5 to 1.5:0.5 to 1.5:1 to 3} in terms of a volume ratio.In addition, it is more preferable that the mixing ratio in the solventbe 0.8 to 1.2:0.8 to 1.2:1.5 to 2.5 in terms of a volume ratio, andstill more preferable that the mixing ratio be in a range ofapproximately {PC:EC:DME}={1:1:2}.

When the mixing ratio of the organic solvents is in the above-describedrange, it is possible to more significantly obtain the above-describedeffect of improving the low-temperature characteristics withoutdeteriorating the capacity retention rate at a high temperature or roomtemperature.

More specifically, when the mixing ratio of the propylene carbonate(PC), which is a cyclic carbonate solvent, is equal to or greater thanthe lower limit in the range, when PC having a melting point lower thanthat of EC, and EC are mixed and used, it is possible to significantlyobtain the effect of improving the low-temperature characteristics.

On the other hand, PC has a lower dielectric constant in comparison toEC, and thus it is difficult to increase a concentration of thesupporting salt. Accordingly, when the amount of PC contained is toogreat, there is a possibility that it is difficult to obtain largedischarging capacity. As a result, it is preferable to limit the mixingratio of PC to a value equal to or less than the upper limit of therange.

In addition, in the organic solvent, when the mixing ratio of theethylene carbonate (EC), which is a cyclic carbonate solvent, is equalto or greater than the lower limit of the range, the dielectric constantof the electrolytic solution 50 and solubility for the supporting saltincrease, and discharging capacity of the nonaqueous electrolytesecondary battery becomes large.

On the other hand, EC has a high viscosity, and thus EC is deficient inelectrical conductivity. In addition, EC has a high melting point, andthus when the amount of EC contained is too large, the low-temperaturecharacteristics may deteriorate. Accordingly, it is preferable to limitthe mixing ratio thereof to a value equal to or less than the upperlimit of the range.

In addition, when the mixing ratio of EC in the organic solvents is setto the range, it is possible to suppress an increase in internalresistance under a low-temperature environment.

In addition, in the organic solvent, when the mixing ratio of dimethoxyethane (DME), which is a chain ether solvent, is set to be equal to orgreater than the lower limit of the range, a predetermined amount of DMEhaving a low melting point is included in the organic solvent, and thusthe effect of improving the low-temperature characteristics becomessignificant. In addition, DME has a low viscosity, and thus electricalconductivity is improved. In addition, DME solvates with Li ions, andthus it is possible to obtain large discharging capacity.

On the other hand, DME has a low dielectric constant, and thus it isdifficult to increase a concentration of the supporting salt.Accordingly, when the amount of DME contained is too large, it may bedifficult to obtain large discharging capacity, and thus it ispreferable to limit the mixing ratio of DME to a value equal to or lessthan the upper limit of the range.

In addition, when the mixing ratio of DME in the organic solvents is setto the range, it is possible to suppress voltage drop at initialdischarging.

As the supporting salt used in the electrolytic solution 50, a known Licompound added to the electrolytic solution as the supporting salt inthe nonaqueous electrolyte secondary battery in the related art can beused. Specifically, any one kind of lithium bis(fluorosulfonyl)imide(LiFSI) and lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) may beused alone, or two kinds thereof may be used in combination.

The amount of the supporting salt contained in the electrolytic solution50 can be determined in consideration of the kind of supporting salt andin consideration of the kind of the following positive electrode activematerial. In this embodiment, the supporting salt is contained in theamount of 0.6 to 1.4 (mol/L) in terms of a total amount of LiFSI orLiTFSI. As the supporting salt in the electrolytic solution 50, thelithium compound is included in a molar ratio of the range, sufficientdischarging capacity is significantly obtained in a broad temperaturerange including a low-temperature environment, and thus batterycharacteristics are improved.

Furthermore, when a concentration of the supporting salt in theelectrolytic solution 50 exceeds the upper limit of the range, it isdifficult to obtain discharging capacity. When the concentration is lessthan the lower limit, internal resistance greatly increases.Accordingly, it is preferable that the concentration of the supportingsalt in the electrolytic solution 50 be set to the range whenconsidering that if the concentration is too high or too low, an adverseinfluence may be exerted on the battery characteristics.

In addition, it is preferable that LiFSI be used alone as the supportingsalt used in the electrolytic solution 50, and 0.6 to 1.4 (mol/L) ofLiFSI be contained in the electrolytic solution 50 when considering thatit is possible to suppress voltage drop at initial discharging, it isalso possible to improve discharging characteristics under alow-temperature environment, and thus sufficient discharging capacity isobtained in a broad temperature range. LiFSI is excellent inconductivity, and thus the above-described effect becomes moresignificant.

In this embodiment, first, the organic solvent used in the electrolyticsolution 50 is set to have the above-described composition. According tothis, the viscosity of the electrolytic solution is prevented fromrising under a low-temperature environment of −30° C. to −40° C., andthus it is possible to suppress migration of electric charges from beinghindered. According to this, the discharging characteristics under thelow-temperature environment are improved, and thus it is possible toretain sufficient discharging capacity in a broad temperature range.

In addition, in this embodiment, as the supporting salt contained in theelectrolytic solution 50, when employing a configuration in which thelithium compound is included in the molar ratio of the range, theabove-described effect is more significantly obtained, and thus thebattery characteristics are further improved.

In addition, in this embodiment, in addition to the optimization of thecomposition of the electrolytic solution 50, when the negative electrode20, of which details will be described later, contains any one negativeelectrode active material between SiO_(X) (0≤X<2) in which at least apart of a surface is covered with carbon and a Li—Al alloy, thedischarging characteristics under the low-temperature environment areimproved, and the effect of retaining sufficient discharging capacity ina broad temperature range becomes more significant.

[Positive Electrode]

As the positive electrode 10, a positive electrode known in this fieldcan be used without particular limitation as long as the positiveelectrode contains a positive electrode active material composed of alithium-manganese oxide. In addition, a material obtained by mixingpolyacrylic acid as a binding agent and graphite as a conductiveauxiliary agent in addition to the positive electrode active materialcan be used as the positive electrode 10.

Examples of the positive electrode active material contained in thepositive electrode 10 include lithium-manganese oxides such as LiMn₂O₄and Li₄Mn₅O₁₂, which have a spinel-type crystal structure. Among thelithium-manganese oxides, a lithium-manganese oxide such asLi_(1+x)Co_(y)Mn_(2−x−y)O₄(0≤x≤0.33, 0<y≤0.2), in which Mn is partiallysubstituted with Co, is more preferable. In this manner, when using thepositive electrode active material in which a transition metal elementsuch as Co and Ni is added to the lithium-manganese oxide and partialsubstitution with a transition metal element is performed, thedischarging characteristics are further improved.

In this embodiment, when using the positive electrode active materialcomposed of the lithium-manganese oxide having the above-describedcomposition in the positive electrode 10, the dischargingcharacteristics under a low-temperature environment are improved, andthe effect of obtaining sufficient discharging capacity in a broadtemperature range becomes more significant. As a result, the batterycharacteristics are further improved.

In addition, in this embodiment, not only one kind of lithium-manganeseoxide but also a plurality of kinds of lithium-manganese oxides may becontained as the positive electrode active material.

In addition, in a case of using a granular positive electrode activematerial composed of the above-described material, a particle size (D50)is not particularly limited, and for example, a particle size of 0.1 to100 μm is preferable, and a particle size of 1 to 10 μm is morepreferable.

If the particle size (D50) of the positive electrode active material isless than the lower limit of the above-described preferable range, whenthe nonaqueous electrolyte secondary battery is exposed to a hightemperature, reactivity increases, and thus handling becomes difficult.In addition, when the particle size exceeds the upper limit thereof,there is a concern that a discharging rate may decrease.

Furthermore, the “particle size (D50) of the positive electrode activematerial” in the invention is a particle size measured by a laserdiffraction method known in the related art and represents a mediansize.

The amount of the positive electrode active material in the positiveelectrode 10 is determined in consideration of discharging capacity andthe like required for the nonaqueous electrolyte secondary battery 1,and 50 to 95 mass % is preferable. When the amount of the positiveelectrode active material is equal to or more than the lower limit ofthe preferable range, it is easy to obtain sufficient dischargingcapacity. When the amount of the positive electrode active material isequal to or less than the upper limit of the preferable range, it iseasy to mold the positive electrode 10.

The positive electrode 10 may contain a conductive auxiliary agent(hereinafter, the conductive auxiliary agent used in the positiveelectrode 10 may be referred to as a “positive electrode conductiveauxiliary agent”).

Examples of the positive electrode conductive auxiliary agent includecarbonaceous materials such as furnace black, Ketjen black, acetyleneblack, and graphite.

As the positive electrode conductive auxiliary agent, theabove-described carbonaceous materials may be used alone or incombination of two or more kinds thereof.

In addition, the amount of the positive electrode conductive auxiliaryagent in the positive electrode 10 is preferably 4 to 40 mass %, andmore preferably 10 to 25 mass %. When the amount of the positiveelectrode conductive auxiliary agent is equal to or greater than thelower limit of the preferable range, it is easy to obtain sufficientconductivity. In addition, in a case of molding the electrode in apellet shape, molding becomes easy. On the other hand, when the amountof the positive electrode conductive auxiliary agent in the positiveelectrode 10 is equal to or less than the upper limit of the preferablerange, it is easy to obtain sufficient discharging capacity in thepositive electrode 10.

The positive electrode 10 may contain a binder (hereinafter, the binderused in the positive electrode 10 may be referred to as a “positiveelectrode binder”).

As the positive electrode binder, a material known in the related artmay be used, and examples thereof include polytetrafluoroethylene(PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR),polyacrylic acid (PA), carboxymethyl cellulose (CMC), polyvinyl alcohol(PVA), and the like. Among these, the polyacrylic acid is preferable,and a cross-linking type polyacrylic acid is more preferable.

In addition, as the positive electrode binder, the above-describedmaterials may be used alone or in combination of two or more kindsthereof.

Furthermore, in a case of using the polyacrylic acid as the positiveelectrode binder, it is preferable that the polyacrylic acid be adjustedin advance to pH 3 to pH 10. In the pH adjustment in this case, forexample, an alkali metal hydroxide such as lithium hydroxide, or analkali-earth metal hydroxide such as magnesium hydroxide can be used.

The amount of the positive electrode binder in the positive electrode 10may be set, for example, to 1 to 20 mass %.

The size of the positive electrode 10 is determined in correspondencewith the size of the nonaqueous electrolyte secondary battery 1.

In addition, the thickness of the positive electrode 10 is alsodetermined in correspondence with the size of the nonaqueous electrolytesecondary battery 1. In a case where the nonaqueous electrolytesecondary battery 1 is a coin type for back-up of various electronicapparatuses, for example, the thickness may be approximately 300 to 1000μm.

The positive electrode 10 may be manufactured by a manufacturing methodknown in the related art.

Examples of a method of manufacturing the positive electrode 10 includea method in which the positive electrode active material, and at leastone of the positive electrode conductive auxiliary agent and thepositive electrode binder added as necessary, are mixed to obtain apositive electrode mixture, and the positive electrode mixture iscompression-molded into an arbitrary shape.

A pressure during the compression molding is determined in considerationof the kind of positive electrode conductive auxiliary agent and thelike, and for example, can be set to 0.2 to 5 ton/cm².

As the positive electrode current collector 14, a material known in therelated art can be used, and examples thereof include a conductive resinadhesive including carbon as a conductive filler, and the like.

[Negative Electrode]

The negative electrode 20 used in this embodiment includes SiO_(X)(0≤X<2) in which at least a part of a surface is covered with carbon, ora Li—Al alloy as the negative electrode active material. As the negativeelectrode 20, a material obtained by additionally mixing an appropriatebinder, a polyacrylic acid as a binding agent, graphite as a conductiveauxiliary agent, and the like in addition to the negative electrodeactive material can also be used.

Examples of the negative electrode active material used in the negativeelectrode 20 include a material composed of SiO or SiO₂, that is, asilicon oxide expressed by SiO_(x) (0≤X<2). When using the silicon oxidehaving the composition as the negative electrode active material, it ispossible to use the nonaqueous electrolyte secondary battery 1 at a highvoltage, and cycle characteristics are improved.

In addition, the negative electrode 20 may further contain at least oneamong carbon, an alloy-based negative electrode active material otherthan the Li—Al alloy, Si, WO₂, and WO₃ as the negative electrode activematerial in addition to the SiO_(x) (0≤X<2) or the Li—Al alloy.

When using the material as the negative electrode active material in thenegative electrode 20, a reaction between the electrolytic solution 50and the negative electrode 20 is suppressed during a charging anddischarging cycle, and thus a decrease in capacity can be prevented. Asa result, the cycle characteristics are improved.

In addition, when the negative electrode 20 includes a negativeelectrode active material composed of SiO_(x) (0≤X<2) in which at leasta part of a surface of is covered with carbon (C), or a Li—Al alloy,conductivity of the negative electrode 20 is improved, and thus anincrease in internal resistance under the low-temperature environment issuppressed. According to this, voltage drop at initial discharging issuppressed, and thus it is possible to further stabilize the dischargingcharacteristics. Furthermore, in a case of using the SiO_(x) (0≤X<2) asthe negative electrode active material, at least a part of a surface ofparticles composed of SiO_(x) (0≤X<2) may be covered with carbon.However, it is preferable that the entirety of the surface be coveredwith carbon when considering that the above-described effect becomesmore significant.

In this embodiment, as described above, first, the composition of theelectrolytic solution 50 is optimized, and when employing the negativeelectrode 20 that includes a negative electrode active material composedof SiO_(x) (0≤X<2) in which a surface is covered with carbon or anegative electrode active material composed of a Li—Al alloy, it ispossible to improve the discharging characteristics under alow-temperature environment of −30° C. to −40° C. According to this, itis possible to realize the nonaqueous electrolyte secondary battery 1 inwhich excellent charging and discharging characteristics are obtained ina broad temperature range.

Furthermore, a method of covering a particle surface of SiO_(x) (0≤X<2)with carbon is not particularly limited, and examples thereof include aphysical vapor deposition method (PVD) using a gas including an organicmaterial such as methane and acetylene, a chemical vapor depositionmethod (CVD), and the like.

In a case of using the SiO_(x) (0≤X<2) as the negative electrode activematerial, for example, the particle size (D50) is preferably 0.1 to 30μm, and more preferably 1 to 10 μm without particular limitation. Whenthe particle size (D50) of the negative electrode active material is inthe above-described range, when expansion or contraction of the negativeelectrode occurs during charging and discharging of the nonaqueouselectrolyte secondary battery, conductivity is retained. As a result, adecrease in charging and discharging characteristics such as cyclecharacteristics is suppressed. When the particle size (D50) of thenegative electrode active material is less than the lower limit of thepreferable range, for example, when the nonaqueous electrolyte secondarybattery is exposed to a high temperature, reactivity increases, and thushandling becomes difficult. In addition, when the particle size exceedsthe upper limit thereof, there is a concern that a discharging rate maydecrease. In addition, the particle size (D50) of the negative electrodeactive material (SiO_(x) (0≤X<2)) in this specification represents aparticle size in a state in which at least a part of a surface of theSiO_(x) (0≤X<2) is covered with carbon.

In addition, in this embodiment, it is more preferable that the negativeelectrode active material in the negative electrode 20 include both oflithium (Li) and SiO_(X) (0≤X<2), and a molar ratio (Li/SiO_(X)) thereofbe in a range of 3.7 to 4.9. In this manner, when the negative electrodeactive material includes both of Lithium (Li) and SiO_(X), and the molarratio thereof is set in the above-described range, it is possible toobtain an effect of preventing charging abnormality, and the like. Inaddition, even when the nonaqueous electrolyte secondary battery 1 isused or stored for a long period of time under a high-temperatureenvironment, the discharging capacity does not decrease, and thusstorage characteristics are improved.

When the molar ratio (Li/SiO_(X)) is less than 3.7, Li is deficient, andthus Li deficiency occurs in use or storage for a long period of timeunder a high-temperature environment, and thus the discharging capacitydecreases.

On the other hand, when the molar ratio (Li/SiO_(X)) exceeds 4.9, Li isexcessive, and thus charging abnormality may occur. In addition, metalLi remains without being trapped in SiO_(X), and thus there is apossibility that internal resistance increases, and thus the dischargingcapacity may decrease.

In addition, in this embodiment, it is more preferable that the molarratio (Li/SiO_(X)) in the above-described range be set by selecting amore appropriate range in correspondence with the kind of positiveelectrode active material contained in the positive electrode 10. Forexample, in a case of using lithium titanate as the positive electrodeactive material, it is more preferable that the molar ratio (Li/SiO_(X))in the negative electrode active material be set to a range of 4.0 to4.7. In addition, in a case of using lithium-manganese oxide as thepositive electrode active material, it is more preferable that molarratio (Li/SiO_(X)) in the negative electrode active material be set to arange of 3.9 to 4.9. As described above, when the molar ratio(Li/SiO_(X)) of the negative electrode active material is set to a rangein correspondence with the kind of positive electrode active material,it is possible to obtain an effect of suppressing an increase in initialresistance and of preventing charging abnormality and the like, or it ispossible to more significantly obtain an effect of improving storagecharacteristics without a decrease in discharging capacity even afteruse or storage for a long period of time under a high-temperatureenvironment.

The amount of the negative electrode active material contained in thenegative electrode 20 is determined in consideration of dischargingcapacity and the like required for the nonaqueous electrolyte secondarybattery 1. For example, the amount of the negative electrode activematerial is preferably equal to or greater than 50 mass %, and morepreferably 60 to 80 mass %.

In the negative electrode 20, when the amount of the negative electrodeactive material composed of the above-described material is equal to orgreater than the lower limit of the preferable range, it is easy toobtain sufficient discharging capacity. In addition, when the amount ofthe negative electrode active material is equal to or less than theupper limit of the preferable range, it is easy to mold the negativeelectrode 20.

The negative electrode 20 may contain a conductive auxiliary agent(hereinafter, the conductive auxiliary agent used in the negativeelectrode 20 may be referred to as a “negative electrode conductiveauxiliary agent”). The negative electrode conductive auxiliary agent isthe same as the positive electrode conductive auxiliary agent.

The negative electrode 20 may contain a binder (hereinafter, the binderused in the negative electrode 20 may be referred to as a “negativeelectrode binder”).

Examples of the negative electrode binder include polyvinylidenefluoride (PVDF), styrene-butadiene rubber (SBR), polyacrylic acid (PA),carboxymethyl cellulose (CMC), polyimide (PI), polyimide amide (PAI),and the like. Among these, the polyacrylic acid is preferable, and across-linking type polyacrylic acid is more preferable.

In addition, as the negative electrode binder, the above-describedmaterials may be used alone or in combination of two or more kindsthereof. In addition, in a case of using the polyacrylic acid as thenegative electrode binder, it is preferable that the polyacrylic acid beadjusted in advance to pH 3 to pH 10. For example, the pH adjustment inthis case can be performed through addition of an alkali metal hydroxidesuch as lithium hydroxide, or an alkali-earth metal hydroxide such asmagnesium hydroxide.

For example, the amount of the negative electrode binder contained inthe negative electrode 20 may be set to 1 to 20 mass %.

Furthermore, the size and the thickness of the negative electrode 20 arethe same as the size and the thickness of the positive electrode 10.

In addition, in the nonaqueous electrolyte secondary battery 1illustrated in FIG. 1, lithium foil 60 is provided on a surface of thenegative electrode 20, that is, between the negative electrode 20 andthe separator 30 to be described later.

Examples of a method of manufacturing the negative electrode 20 includea method in which the above-described material is used as the negativeelectrode active material, and the negative electrode conductiveauxiliary agent such as graphite and/or the negative electrode binderare mixed as necessary to prepare a negative electrode mixture, and thenegative electrode mixture is compression-molded into an arbitraryshape.

A pressure during the compression molding is determined in considerationof the kind of negative electrode conductive auxiliary agent and thelike, and for example, can be set to 0.2 to 5 ton/cm².

In addition, the same material as the positive electrode currentcollector 14 can be used for the negative electrode current collector24.

[Separator]

The separator 30 is interposed between the positive electrode 10 and thenegative electrode 20, and as the separator 30, an insulating film,which has large ion permeability, is excellent in heat resistance, andhas a predetermined mechanical strength, is used.

As the separator 30, a separator formed from a material used in aseparator of a nonaqueous electrolyte secondary battery in the relatedart and satisfies the above-described characteristics may be appliedwithout any limitation. Examples of the material include glass such asalkali glass, borosilicate glass, quartz glass, and lead glass,non-woven fabric or fiber formed from a resin such as polyphenylenesulfide (PPS), polyether ether ketone (PEEK), polyethylene terephthalate(PET), polyamide-imide (PAI), polyamide, polyimide (PI), aramid,cellulose, a fluorine resin, and a ceramic resin, and the like. Amongthese, as the separator 30, it is more preferable to use the non-wovenfabric formed from glass fiber. The glass fiber is excellent inmechanical strength and has large ion permeability, and thus the glassfiber reduces internal resistance. Accordingly, it is possible toimprove discharging capacity.

The thickness of the separator 30 is determined in consideration of thesize of the nonaqueous electrolyte secondary battery 1, a material ofthe separator 30, and the like, and for example, can be set toapproximately 5 to 300 μm.

[Capacity Balance between Negative Electrode and Positive Electrode]

In the nonaqueous electrolyte secondary battery 1 of this embodiment, itis preferable that capacity balance {negative electrode capacity(mAh)/positive electrode capacity (mAh)}, which is expressed by capacityof the negative electrode 20 and capacity of the positive electrode 10,be in a range of 1.56 to 2.51.

When the capacity balance between the negative electrode 20 and thepositive electrode 10 is set to the above-described range, apredetermined margin can be secured for the capacity on the negativeelectrode side. Accordingly, for example, even when decomposition of thenegative electrode active material rapidly progresses due to a batteryreaction, it is possible to secure the negative electrode capacity equalto or greater than a constant value. Accordingly, even when thenonaqueous electrolyte secondary battery 1 is used and stored for a longperiod of time under a strict high-temperature and high-humidityenvironment, a decrease in discharging capacity is suppressed, and thusit is possible to obtain an effect of improving storage characteristics.

When the capacity balance between the negative electrode 20 and thepositive electrode 10 is less than 1.56, deterioration increases duringuse for a long period of time under a high-temperature environment, andthus capacity retention becomes difficult. On the other hand, when thecapacity balance between the negative electrode 20 and the positiveelectrode 10 exceeds 2.51, it is difficult to obtain sufficientdischarging capacity.

In the nonaqueous electrolyte secondary battery 1 of this embodiment,the composition of the electrolytic solution 50 is optimized asdescribed above. In addition, as the negative electrode 20, an electrodeincluding at least a negative electrode active material composed ofSiO_(X) (0≤X<2) in which at least a part of a surface is covered withcarbon or a negative electrode active material composed of a Li—Al alloyis used. In addition, the capacity balance between the negativeelectrode 20 and the positive electrode 10 is set to the above-describedappropriate range. According to this, the discharging characteristicsunder a low-temperature environment are improved, and thus sufficientdischarging capacity can be retained in a broad temperature range. Inaddition, storage characteristics become excellent.

<Other Aspects of Nonaqueous Electrolyte Secondary Battery>

In this embodiment, as an embodiment of the nonaqueous electrolytesecondary battery, description has been given of the nonaqueouselectrolyte secondary battery having a coin-type structure including theaccommodation container obtained by crimping the positive electrodecasing and the negative electrode casing formed from stainless steel,but the invention is not limited thereto. For example, it is alsopossible to employ a nonaqueous electrolyte secondary battery having astructure in which an opening of a ceramic container main body is sealedwith a ceramic lid through a heating treatment such as seam weldingusing a metallic sealing member.

In addition, for example, the configuration related to the invention isalso applicable to an electrochemical cell such as a lithium ioncapacitor.

<Use of Nonaqueous Electrolyte Secondary Battery>

As described above, according to the nonaqueous electrolyte secondarybattery 1 of this embodiment, the discharging characteristics under alow-temperature environment is excellent, and sufficient dischargingcapacity is obtained in a broad temperature range. In addition, evenwhen the nonaqueous electrolyte secondary battery 1 is used and storedfor a long period of time under a strict temperature and humidityenvironment, high discharging capacity can be retained, and storagecharacteristics are excellent. Accordingly, the nonaqueous electrolytesecondary battery 1 is appropriately used as a back-up power supplyhaving a voltage value, for example, 2 to 3 V.

<Operational Effect>

As described above, according to the nonaqueous electrolyte secondarybattery 1 as an embodiment of the invention, in the electrolyticsolution 50, an organic solvent containing propylene carbonate (PC),ethylene carbonate (EC), and dimethoxy ethane (DME) in a range of{PC:EC:DME}={0.5 to 1.5:0.5 to 1.5:1 to 3} in terms of a volume ratio isused, and a supporting salt containing at least one of lithiumbis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in a total amount of 0.6 to 1.4 (mol/L) isused. In addition, the negative electrode 20, which includes SiO_(X)(0≤X<2) in which at least a part of a surface is covered with carbon, ora Li—Al alloy as a negative electrode active material, is used.

As described above, the composition of the organic solvent and thesupporting salt used in the electrolytic solution is optimized, and theabove-described material is used as the negative electrode activematerial in the negative electrode. Accordingly, it is possible toimprove the discharging characteristics under a low-temperatureenvironment. That is, even when the nonaqueous electrolyte secondarybattery 1 is used or stored under a low-temperature environment of −30°C. to −40° C., excellent discharging characteristics are obtained, andthus it is possible to retain sufficient discharging capacity in a broadtemperature range.

Accordingly, it is possible to provide the nonaqueous electrolytesecondary battery 1 in which the battery characteristics do notdeteriorate even under the low-temperature environment and thusexcellent charging and discharging characteristics are obtained in abroad temperature range, and excellent storage characteristics areprovided.

EXAMPLES

Next, the invention will be described in more detail with reference toExamples and Comparative Examples. However, a range of the invention isnot limited by Examples. The nonaqueous electrolyte secondary batteryrelated to the invention may be executed by making appropriatemodifications in a range not departing from the gist of the invention.

<Adjustment of Electrolytic Solution and Preparation of NonaqueousElectrolyte Secondary Battery>

Examples 1 and 2, and Comparative Example 1

In Example 1, a coin-type nonaqueous electrolyte secondary batteryillustrated in FIG. 1 was prepared as an electrochemical cell.Furthermore, in this example, a nonaqueous electrolyte secondary batterywas prepared by adjusting an electrolytic solution having a compositionillustrated in Table 1, and by using Li_(1.14)Co_(0.06)Mn_(1.80)O₄ as apositive electrode active material, and SiO, in which the entirety ofsurface is covered with carbon, as a negative electrode active material.In this example, a coin-type (920 size) nonaqueous electrolyte secondarybattery (lithium secondary battery), in which an external shape and thethickness in the cross-sectional view illustrated in FIG. 1 wererespectively set to 9.5 mm and 2.0 mm, was prepared.

(Adjustment of Electrolytic Solution)

First, the organic solvent was adjusted in accordance with a mixingratio (volume %) illustrated in Table 1, and the supporting salt wasdissolved in the organic solvent to adjust the electrolytic solution 50.At this time, as the organic solvent, propylene carbonate (PC), ethylenecarbonate (EC), and dimethoxy ethane (DME) were mixed in a ratio of{PC:EC:DME}={1:1:2} in terms of a volume ratio to adjust a mixedsolvent. Then, as the supporting salt, lithium bis(fluorosulfonyl)imide(LiFSI) (Example 2) or lithium bis(trifluoromethane sulfonyl) imide(LiTFSI) (Example 1, Comparative Example 1) was dissolved in the mixedsolvent that was obtained in a concentration illustrated in Table 1 toobtain the electrolytic solution 50.

(Preparation of Battery)

With regard to the positive electrode 10, first, graphite as aconductive auxiliary agent and polyacrylic acid as a binding agent weremixed in commercially available lithium-manganese oxide(Li_(1.14)Co_(0.06)Mn_(1.80)O₄) in a ratio of lithium-manganeseoxide:graphite:polyacrylic acid=90:8:2 (mass ratio) to obtain a positiveelectrode mixture.

Then, 98.6 mg of positive electrode mixture that was obtained wascompressed with a compression force of 2 ton/cm² to perform compressionmolding into a disc-shaped pellet having a diameter of 8.9 mm.

Next, the pellet (positive electrode 10) that was obtained was bonded toan inner surface of the positive electrode casing 12 formed fromstainless steel (NAS64: t=0.20 mm) using a conductive resin adhesivecontaining carbon to integrate the pellet and the positive electrodecasing 12, thereby obtaining a positive electrode unit. Then, thepositive electrode unit was decompressed, heated, and dried underconditions of 120° C. for 11 hours in the air.

In addition, a sealing agent was applied to an inner side surface of theopening 12 a of the positive electrode casing 12 in the positiveelectrode unit.

Next, with regard to the negative electrode 20, first, a SiO powder, inwhich carbon (C) is formed the entirety of a surface, was prepared as anegative electrode active material. In addition, graphite as aconductive auxiliary agent and polyacrylic acid as a binding agent weremixed to the negative electrode active material in a ratio of 54:44:2(mass ratio) to obtain a negative electrode mixture.

Next, 15.1 mg of negative electrode mixture that was obtained wascompression-molded with a compression force of 2 ton/cm² into adisc-shaped pellet having a diameter of 6.7 mm.

Next, the pellet (negative electrode 20) that was obtained was bonded toan inner surface of the negative electrode casing 22 formed fromstainless steel (SUS304-BA: t=0.20 mm) using a conductive resin adhesivecontaining carbon as a conductive filler to integrate the pellet and thenegative electrode casing 22, thereby obtaining a negative electrodeunit. Then, the negative electrode unit was decompressed, heated, anddried under conditions of 160° C. for 11 hours in the air.

In addition, lithium foil 60 punched with a diameter of 6.1 mm and athickness of 0.38 mm was additionally compressed onto the negativeelectrode 20 having a pellet shape to prepare a lithium-negativeelectrode stacked electrode.

As described above, in this example, the positive electrode casing 12was provided with a function of a positive electrode current collector,and the negative electrode casing 22 was provided with a function of anegative electrode current collector without providing the positiveelectrode current collector 14 and the negative electrode currentcollector 24 illustrated in FIG. 1. In this manner, a nonaqueouselectrolyte secondary battery was prepared.

In addition, in preparation of the electrodes, capacity balance{negative electrode capacity (mAh)/positive electrode capacity (mAh)},which is expressed by the capacity of the negative electrode 20 and thecapacity of the positive electrode 10, was adjusted to a valueillustrated in Table 1.

Next, non-woven fabric formed from glass fiber was dried and was punchedinto a disc shape having a diameter of 7 mm to prepare the separator 30.In addition, the separator 30 was mounted on the lithium foil 60compressed onto the negative electrode 20, and the gasket 40 formed frompolypropylene was disposed over an opening of the negative electrodecasing 22.

Next, the positive electrode casing 12 and the negative electrode casing22 were filled with the electrolytic solution 50, which was adjusted inthe above-described procedure, in a total amount of 40 μL for eachbattery.

Next, the negative electrode unit was caulked to the positive electrodeunit so that the separator 30 came into contact with the positiveelectrode 10. In addition, the positive electrode casing 12 and thenegative electrode casing 22 were sealed through fitting of the openingof the positive electrode casing 12, and the resultant sealed body wasleft still at 25° C. for seven days to obtain a nonaqueous electrolytesecondary battery of Examples 1 and 2, and Comparative Example 1.

TABLE 1 Internal resistance Capacity balance Electrolytic solution (Ω)Electrode (Negative electrode Concen- Room Positive electrode Negativeelectrode capacity/positive Organic Support- tration temper- activematerial active material electrode capacity) solvent Ratio ing salt(mol/L) ature Example 1 Li_(1.14)Co_(0.06)Mn_(1.80)O₄ SiO (covered 2.02PC + EC + 1:1:2 LiTFSI 1 24.1 Example 2 with carbon) DME LiFSI 1 19.8Comparative LiTFSI 1.4 40.6 Example 1 Discharging capacity (mAh)Discharging capacity (mAh) Internal (current: 5 μA) (current: 25 μA)resistance Room Capacity Room Capacity (Ω) temper- retention temper-retention −40° C. ature −40° C. rate (%) ature −40° C. rate (%) Example1 392.9 7.94 4.97 69 7.94 3.5 46 Example 2 176.5 — — — 7.46 3.72 50Comparative 1265.9 7.94 0.21 3 — — — Example 1

Comparative Example 2

In Comparative Example 2, a nonaqueous electrolyte secondary battery wasprepared in the same procedure as in Example 1 except that in thebattery preparation conditions in Example 1, the positive electrodeactive material used in the positive electrode was changed to a materialcomposed of Li₄Mn₅O₁₂, a negative electrode including a negativeelectrode active material composed of SiO in which a surface is notcovered with carbon was used as the negative electrode, the capacitybalance {negative electrode capacity (mAh)/positive electrode capacity(mAh)}, which is expressed by the capacity of the negative electrode andthe capacity of the positive electrode, was adjusted to a valueillustrated in Table 2 during preparation of the respective electrodes.

TABLE 2 Internal resistance Capacity balance Electrolytic solution (Ω)Electrode (Negative electrode Concen- Room Positive electrode Negativeelectrode capacity/positive Organic Support- tration temper- activematerial active material electrode capacity) solvent Ratio ing salt(mol/L) ature Example 1 Li_(1.14)Co_(0.06)Mn_(1.80)O₄ SiO (covered 2.02PC + EC + 1:1:2 LiTFSI 1 24.1 with carbon) DME Comparative Li₄Mn₅O₁₂ SiO(not covered 1 65.2 Example 2 with carbon) Discharging capacity (mAh)Discharging capacity (mAh) Internal (current: 5 μA) (current: 15 μA)resistance Room Capacity Room Capacity (Ω) temper- retention temper-retention −40° C. ature −40° C. rate (%) ature −40° C. rate (%) Example1 392.9 7.94 4.97 69 7.94 4.04 56 Comparative 1293.2 5.55 0.32 6 5.550.1 2 Example 2

Experiment Example 1

In Experiment Example 1, a nonaqueous electrolyte secondary battery wasprepared in the same procedure as in Example 1 except that in thebattery preparation conditions in Example 1, the capacity balance{negative electrode capacity (mAh)/positive electrode capacity (mAh)},which is expressed by the capacity of the negative electrode and thecapacity of the positive electrode, was adjusted to a value illustratedin Table 3 during preparation of the respective electrodes.

TABLE 3 Internal resistance Capacity balance (Ω) Electrode (Negativeelectrode Electrolytic solution Room Positive electrode Negativeelectrode capacity/positive Organic Support- Concen- temper- activematerial active material electrode capacity) solvent Ratio ing salttration ature Example 1 Li_(1.14)Co_(0.06)Mn_(1.80)O₄ SiO (covered 2.03PC + EC + 1:1:2 LiTFSI 1 mol/L 24.1 Experiment with carbon) 1.53 DME98.1 Example 1 Discharging capacity (mAh) Discharging capacity (mAh)Internal (current: 5 μA) (current: 10 μA) resistance Room Capacity RoomCapacity (Ω) temper- retention temper- retention −40° C. ature −40° C.rate (%) ature −40° C. rate (%) Example 1 392.9 7.94 4.97 69 7.94 4.5764 Experiment 1499.7 1.99 0.57 29 1.99 0.22 11 Example 1

Examples 3 to 6, and Comparative Examples 3 to 6

In Examples 3 to 6, and Comparative Examples 3 to 6, a nonaqueouselectrolyte secondary battery was prepared in the same procedure as inExample 1 except that in the battery preparation conditions in Example1, a concentration of the supporting salt (LiFSI) included in theelectrolytic solution was changed to a value illustrated in Table 4, andthe capacity balance {negative electrode capacity (mAh)/positiveelectrode capacity (mAh)} was adjusted to a value illustrated in Table4. Furthermore, in Examples 3 to 6, and Comparative Examples 3 to 6, acoin-type (621 size) nonaqueous electrolyte secondary battery in whichthe external shape and the thickness in the cross-sectional viewillustrated in FIG. 1 were respectively set to 6.8 mm and 2.1 mm wasprepared.

TABLE 4 Capacity balance Electrode (Negative Discharging capacity (mAh)Positive Negative electrode Electrolytic solution (current: 15 μA)electrode electrode capacity/positive Concen- Room Capacity activeactive electrode Organic Support- tration temper- retention materialmaterial capacity) solvent Ratio ing salt (mol/L) ature −40° C. rate (%)Example 3 Li_(1.14)Co_(0.06)Mn_(1.80)O₄ SiO (covered 2.17 PC + EC + DME1:1:2 LiFSI 0.6 3.435 1.167 34.0 Example 4 with carbon) 0.8 3.403 0.96528.4 Example 5 1.0 3.407 1.024 30.1 Example 6 1.2 3.416 0.967 28.3Comparative 1.4 3.441 0.740 21.5 Example 3 Comparative 1.6 3.435 0.58917.1 Example 4 Comparative 1.8 3.440 0.417 12.1 Example 5 Comparative2.0 3.348 0.242 7.2 Example 6

Examples 7 to 11, and Comparative Examples 7 to 9

In Examples 7 to 11, and Comparative Examples 7 to 9, a nonaqueouselectrolyte secondary battery was prepared in the same procedure as inExample 1 except that in the battery preparation conditions in Example1, the negative electrode active material used in the negative electrodewas changed to a material composed of a Li—Al alloy, and theconcentration of the supporting salt (LiFSI) included in theelectrolytic solution was changed to a value illustrated in Table 5.Furthermore, in Examples 7 to 11, and Comparative Examples 7 to 9, acoin-type (414 size) nonaqueous electrolyte secondary battery in whichthe external shape and the thickness in the cross-sectional viewillustrated in FIG. 1 were respectively set to 4.8 mm and 1.4 mm wasprepared.

Comparative Example 10

In Comparative Example 10, a nonaqueous electrolyte secondary batterywas prepared in the same conditions and the same procedure as inExamples 7 to 11, and Comparative Examples 7 to 9 except that an organicsolvent including triethylene glycol (TEG) and 1,2-diethoxy ethane (DEE)in a ratio of 1:1 was used the electrolytic solution, and LiTFSI wasadded as the supporting salt.

TABLE 5 Electrode Discharging capacity (mAh) Positive NegativeElectrolytic solution (current: 5 μA) electrode electrode Concen- RoomCapacity active active Organic Support- tration temper- retentionmaterial material solvent Ratio ing salt (mol/L) ature −40° C. rate (%)Example 7 Li_(1.14)Co_(0.06)Mn_(1.80)O₄ Li—Al PC + EC + DME 1:1:2 LiFSI0.6 1.080 0.656 60.8 Example 8 alloy 0.8 1.080 0.630 58.4 Example 9 1.01.108 0.661 59.7 Example 10 1.2 1.071 0.384 35.8 Example 11 1.4 1.1000.354 32.1 Comparative 1.6 1.099 0.261 23.7 Example 7 Comparative 1.81.068 0.094 8.8 Example 8 Comparative 2.0 1.078 0.054 5.0 Example 9Comparative TEG + DEE 1:1 LiTFSI 2.0 0.992 0.316 31.9 Example 10

<Evaluation Method>

The following evaluation test was carried out with respect to thenonaqueous electrolyte secondary batteries in Examples 1 to 11,Experiment Example 1, and Comparative Examples 1 to 10 which wereobtained in the procedure.

[Internal Resistance Test]

The internal resistance of the nonaqueous electrolyte secondary batteryof Examples 1 and 2, Experiment Example 1, and Comparative Examples 1and 2 was measured by using a commercially available LCR meter with an Rfunction, and a value at an alternating current of 1 kHz was measured.At this time, the internal resistance of the nonaqueous electrolytesecondary batteries of respective examples was measured under two kindsof temperature conditions, that is, under a room temperature (25° C.)environment and under a low-temperature (−40° C.) environment, andresults are illustrated in Table 1 to Table 3.

[Discharging Capacity and Capacity Retention Rate]

With respect to the nonaqueous electrolyte secondary batteries ofExamples 1 to 11, Experiment Example 1, and Comparative Examples 1 to10, a low-temperature storage test to be described below was performedto evaluate the capacity retention rate under the low-temperatureenvironment.

Specifically, first, the nonaqueous electrolyte secondary batteries ofExamples 1 to 11, Experiment Example 1, and Comparative Examples 1 to 10were discharged under an environment of 25° C. with a constant currentof 5 μA and 25 μA (discharging current) until reaching a voltage of 2.0V, and then, a voltage of 3.1 V was applied to the nonaqueouselectrolyte secondary batteries for 72 hours under an environment of 25°C. Then, capacity when performing discharging with a constant current of5 μA and 25 μA (discharging current) under an environment of 25° C.until reaching a voltage of 2.0 V was measured, and the resultant valueis illustrated in Tables 1 to 3 as discharging capacity (initialcapacity: mAh) under a room temperature.

Next, the nonaqueous electrolyte secondary batteries were left as is for60 days while being exposed to a low-temperature environment of −40° C.by using a constant-humidity and constant-temperature bath.

In addition, with respect to the nonaqueous electrolyte secondarybatteries exposed to the low-temperature environment of theabove-described conditions, capacity when performing discharging with adischarging current of a constant current of 5 μA, and any onedischarging current among 10 μA, 15 μA, and 25 μA under an environmentof 25° C. until reaching a voltage of 2.0 V was measured, and theresultant value is illustrated in Tables 1 to 3 as discharging capacity(capacity after test: mAh) after being left as is at a low temperature(−40° C.), and a capacity retention rate at this time is illustrated inTables 1 to 3.

In the capacity retention rate test in this example, a variation(reduced state) of capacity after a test with respect to the initialcapacity at room temperature (25° C.) was set as an index of thecapacity retention rate under the low-temperature environment.

[Evaluation Result]

As illustrated in Tables 1 to 3, in the nonaqueous electrolyte secondarybatteries of Examples 1 and 2 in which an electrolytic solution obtainedby using a mixed solvent and a supporting salt defined in the invention(the aspect) was used as the organic solvent, and which include apositive electrode including Li_(1.14)Co_(0.06)Mn_(1.80)O₄, which is alithium-manganese oxide, as the positive electrode active material, anda negative electrode including SiO, in which a surface is covered withcarbon, as the negative electrode active material, internal resistanceat room temperature (25° C.) was 19.8 to 24.1 (Ω), and internalresistance at a low temperature (−40° C.) was 176.5 to 392.9 (Ω)). As aresult, it could be seen that the internal resistance was furthersuppressed in comparison to Comparative Examples 1 and 2 (40.6 to 65.2(Ω) at room temperature, and 1265.9 to 1293.2 (Ω) at a low temperature).

In addition, in the nonaqueous electrolyte secondary battery of Example1, when being discharged with a constant current of 5 μA, a capacityretention rate of discharging capacity at a low temperature with respectto discharging capacity at room temperature was 69(%). As a result, itcan be seen that the capacity retention rate is very higher incomparison to Comparative Examples 1 and 2 (3 to 6(%)). In addition,even when being discharged with a constant current of 15 to 25 μA, acapacity retention rate of the nonaqueous electrolyte secondary batteryof Examples 1 and 2 was 46 to 56(%). In Comparative Examples 1 and 2,capacity measurement was impossible due to voltage drop, or even thoughthe capacity measurement was possible, the capacity retention rate wasmerely approximately 2%. From this result, it is apparent that thenonaqueous electrolyte secondary batteries of Examples 1 and 2 havingthe configuration related to the invention have excellent capacityretention rate.

Here, in Comparative Example 1 illustrated in Table 1, a concentrationof the supporting salt (LiTFSI) contained in the electrolytic solutiondeviates a range defined in the invention. Accordingly, it is consideredthat the internal resistance was not suppressed, and the capacityretention rate of the discharging capacity at a low temperature withrespect to the discharging capacity at room temperature was lowered.

On the other hand, Table 2 illustrates evaluation results of Example 1which includes a negative electrode including SiO in which a surface iscovered with carbon as the negative electrode active material, andcapacity balance between the negative electrode and the positiveelectrode is set to 2.02, and Comparative Example 2 which includes anegative electrode including SiO in which a surface is not covered withcarbon as the negative electrode active material, and capacity balancebetween the negative electrode and the positive electrode is set to 1.As illustrated in Table 2, it can be seen that in the nonaqueouselectrolyte secondary battery of Example 1 having the configurationrelated to the invention, the internal resistance is suppressed to avery low value and the capacity retention rate of the dischargingcapacity at a low temperature with respect to the discharging capacityat room temperature is very high in comparison to the nonaqueouselectrolyte secondary battery of Comparative Example 2.

In addition, Table 3 illustrates results of Experiment Example 1 andExample 1. Experiment Example 1 is common to Example 1 in that anelectrolytic solution including the mixed solvent and the supportingsalt defined in the invention (the aspect) is used, and a positiveelectrode including Li_(1.14)Co_(0.06)Mn_(1.80)O₄, which is alithium-manganese oxide, as the positive electrode active material and anegative electrode including SiO in which a surface is covered withcarbon as the negative electrode active material are provided. InExperiment Example 1, only the capacity balance between the negativeelectrode and the positive electrode is changed differently fromExperiment 1. As illustrated in Table 3, in Example 1 in which thecapacity balance {negative electrode capacity (mAh)/positive electrodecapacity (mAh)} between the negative electrode and the positiveelectrode is in a range of 1.56 to 2.51 defined in claim 5 of theinvention, it can be seen that the internal resistance is suppressed toa very low value, and the capacity retention rate of the dischargingcapacity at a low temperature with respect to the discharging capacityat room temperature is very high in comparison to Experiment Example 1in which the capacity balance between the negative electrode and thepositive electrode is out of the above-described range.

In addition, as illustrated in Table 4, in a case where theconcentration of the supporting salt (LiFSI) in the electrolyticsolution varies, in Examples 3 to 6 in which the concentration of LiFSIis in a range of 0.6 to 1.4 mol/L, it can be seen that the capacityretention rate of the discharging capacity at a low temperature is 21.5to 34.0(%), and is higher in comparison to Comparative Examples 3 to 6(7.2 to 17.1(%)).

In addition, as illustrated in Table 5, even in a case where a negativeelectrode containing a Li—Al alloy is used as the negative electrodeactive material instead of SiO in which a surface is covered withcarbon, and the concentration of the supporting salt (LiFSI) in theelectrolytic solution varies, in Examples 7 to 11 in which theconcentration of LiFSI is in a range of 0.6 to 1.4 mol/L, it can be seenthat the capacity retention rate of the discharging capacity at a lowtemperature is 32.1 to 60.8(%), and is higher in comparison toComparative Examples 7 to 9 (5.0 to 23.7(%)). Particularly, in a casewhere the concentration of the supporting salt in the electrolyticsolution is in a range of 0.6 (Example 7) to 1.0 (Example 9) mol/L, itcan be seen that the capacity retention rate of the discharging capacityat a low temperature is 58.4 to 60.8(%) and is significantly excellent.

On the other hand, in Comparative Examples 3 to 6 illustrated in Table4, the concentration of the supporting salt (LiFSI) in the electrolyticsolution deviates from the range defined in the invention (the aspect).Therefore, it is considered that in Comparative Examples 3 to 6, thecapacity retention rate of the discharging capacity at a low temperaturewith respect to the discharging capacity at room temperature is lowered.

In addition, in Comparative Examples 7 to 9 illustrated in Table 5, theconcentration of the supporting salt (LiFSI) in the electrolyticsolution also deviates from the range defined in the invention (theaspect). Therefore, it is considered that in Comparative Examples 7 to9, the capacity retention rate of the discharging capacity at a lowtemperature with respect to the discharging capacity at room temperatureis also lowered.

Furthermore, in Comparative Example 10 illustrated in Table 5, thecomposition of the organic solvent included in the electrolyticsolution, and the concentration of the supporting salt included in theelectrolytic solution are out of the range defined in the invention (theaspect). Therefore, it is considered that in Comparative Example 10, thecapacity retention rate of the discharging capacity at a low temperaturewith respect to the discharging capacity at room temperature is alsolowered.

From the above-described results of Examples, when employing aconfiguration in which the electrolytic solution having the compositiondefined in the invention is used, and the negative electrode includingSiO_(X) (0≤X<2), in which at least a part of a surface is covered withcarbon, is used as the negative electrode active material, it isapparent that it is possible to obtain a nonaqueous electrolytesecondary battery capable of improving discharging characteristics undera low-temperature environment, and capable of retaining sufficientdischarging capacity in a broad temperature range.

INDUSTRIAL APPLICABILITY

According to the nonaqueous electrolyte secondary battery of theinvention, the composition of the organic solvent and the supportingsalt used in the electrolytic solution is optimized, and the negativeelectrode includes at least one of a negative electrode active materialin which a surface is covered with carbon and a negative electrodeactive material composed of a Li—Al alloy. Accordingly, even in a casewhere a nonaqueous electrolyte secondary battery is used or stored undera low-temperature environment of −30° C. to −40° C., excellentdischarging characteristics are obtained, and thus excellent chargingand discharging characteristics are obtained in a broad temperaturerange. Accordingly, when the invention is applied to, for example, anonaqueous electrolyte secondary battery used in fields of variouselectronic apparatuses and the like, it is possible to contribute to aperformance improvement of the various electronic apparatuses and thelike.

What is claimed is:
 1. A nonaqueous electrolyte secondary battery,comprising: a positive electrode that includes a lithium-manganese oxideas a positive electrode active material; a negative electrode thatincludes SiO_(X) (0≤X<2) in which at least a part of a surface iscovered with carbon, or a Li—Al alloy as a negative electrode activematerial; and a low viscosity electrolytic solution that containspropylene carbonate (PC), ethylene carbonate (EC), and dimethoxy ethane(DME) as an organic solvent in a range of {PC:EC:DME}={0.5 to 1.5:0.5 to1.5:1 to 3} in terms of a volume ratio, and lithiumbis(fluorosulfonyl)imide (LiFSI) as a supporting salt in an individualamount of 0.6 to 1.2 (mol/L), where a viscosity of the electrolyticsolution is constant at −30° C. to −40° C.
 2. The nonaqueous electrolytesecondary battery according to claim 1, wherein the positive electrodeincludes at least Li_(1+x)Co_(y)Mn_(2−x−y)O₄(0≤x≤0.33, 0<y≤0.2) as thelithium-manganese oxide used in the positive electrode active material.3. The nonaqueous electrolyte secondary battery according to claim 1,wherein capacity balance {negative electrode capacity (mAh)/positiveelectrode capacity (mAh)}, which is expressed by capacity of thenegative electrode and capacity of the positive electrode, is in a rangeof 1.56 to 2.51.
 4. The nonaqueous electrolyte secondary batteryaccording to claim 1, wherein a particle size (D50) of SiO_(X) (0≤X<2)included in the negative electrode active material and in which at leasta part of a surface is covered with carbon is 0.1 to 30 μm.